1. Field of the Invention
The present invention relates to optical information recording media having a plurality of information recording layers, and an optical information recording method and optical information recording and reproducing devices for recording information thereon.
2. Description of the Related Art
Recent years have witnessed robust developments in optical disks capable of recording and reproducing information signals such as video and audio signals. Among these, optical disks on which two information recording layers are deposited so as to double the recording capacity are already commercially available as read-only DVDs (Digital Versatile Disks). As mentioned in JP 54-130902A, for example, the front recording layer, seen from the side from which the light source is incident, of such optical disks having a plurality of information recording layers is made semitransparent, so that a sufficient amount of light is allowed to reach the recording layers that are farther from the side from which the light is incident.
Phase-change type optical disks are one type of optical disk media capable of high-density recording. Data are recorded on phase-change optical disks by focusing the laser light to a beam spot less than 1 μm in diameter and directing this laser light onto the revolving disk to heat and melt the recording film. The temperature that the recording film reaches and how it cools differs depending on the intensity of the recording laser, and the recording film is phase changed between crystalline and amorphous states.
When the intensity of the laser light is high (referred to as the “peak power level”), the recording film is heated above its melting point and melted and subsequently cools quickly, thereby becoming amorphous. When the laser light is medium intensity (referred to as the “bias power level”), the recording film is kept above the temperature at which it crystallizes but below its melting point, and therefore it is crystallized. Amorphous areas are called marks and crystallized areas are called spaces. The method of recording data by assigning information to the length of the marks and spaces is called “mark-edge recording.” With phase-change optical disks, the recording film can be melted at the peak power level to form marks, regardless of whether the recording film is amorphous or crystalline, and therefore direct overwriting, where old data are erased and new data are recorded simultaneously with a single beam spot, is possible.
Reproduction is carried out by irradiating a laser light weak enough not to cause the recording film to change phase and detecting the intensity of the reflected light by a photodetector. The material used for the recording film and how it is configured with the protection layer allows the reflectance of the amorphous mark portions to be significantly different from the reflectance of the crystalline space portions, so that the difference in the amount of light that is reflected by the mark portions and the space portions can be detected to obtain signals for reproducing the data information.
Numerous proposals have been made for using such phase-change optical disks in the two-layer optical disks described above. For example, JP 2000-311346A proposes to provide the laser light at an ideal recording power for each recording layer because the recording layers differ from one another. In another proposal, JP 2000-260060A teaches a technique for adopting a crystallization promotion film and an optical enhancement film, so that a stable change between amorphous and crystalline states can be generated so as to record information without employing a reflective film on the front recording layer.
However, the prior art mentioned in JP 54-130902A primarily addresses read-only applications, and lacks sufficient investigation into rewritable media with light transmission, absorbance, and reflectance properties adequate for practical applications. The technology disclosed in JP 2000-311346A merely uses recording pulses having identical patterns with respect to a plurality of recording layers and optimizing the peak power at each layer. Furthermore, with the technology that is disclosed in JP 2000-260060A, there is the problem that the optical disk structure becomes complex, and as a consequence additional manufacturing steps are required.
The present invention was derived in light of the foregoing problems, and it is an object thereof to provide an optical information recording medium having a plurality of rewritable information recording layers and to which information signals can be favorably recorded, and a method and a device for favorably recording information signals on an optical information recording medium.